This invention relates to polychloroprene rubber compositions having improved flex crack growth resistance.
Polychloroprene rubber, a homopolymer or copolymer of 2-chlorobutadiene, has traditionally been used to produce articles requiring a combination of heat and ozone resistance coupled with excellent dynamic properties, for example, flex crack growth resistance. In particular, sulfur-modified polychloroprene has long been considered the rubber of choice for articles subjected to repeated flexing, such as automotive belts, automotive boots, air-springs, and motor-mounts. There is, however, a need in the art to provide automotive parts for under-the-hood uses with an even greater reliability than that which is provided by sulfur-modified polychloroprene itself. A typical failure mode under dynamic load is fatigue, which propagates on an initial flaw. Consequently, it is the rate of cut growth which must be improved.
There are two general approaches to upgrading flex-fatigue resistance, by which term is meant prolonging the time to failure after an initial crack has been formed. The first involves modification of the polymer backbone by minimizing failure-initiating polymer defects, or by introducing heat- and/or fatigue-resistant segments. Such an approach is disclosed in U.S. Pat. No. 4,605,705. In the second approach the polymer backbone remains intact and flex-additives are blended with the polymer to produce crack growth-resistant compositions in much the same way as antioxidants and other stabilizers are used to produce polymer compositions having improved resistance to the effects of oxygen, ozone, light, etc. For example, mercaptotolylimidazole has been reported to be an effective flex-growth additive, see Elastomerics, Vol. 117, February 1985. In either case, however, it is to be understood that any attempt to improve flex-fatigue resistance must not result in degradation of other polymer physical properties. Most importantly there must be no decrease in modulus or increase in elongation, and, in addition, Mooney scorch must not be adversely affected.
Blends of polychloroprene and certain ethylene/(meth)acrylate copolymers have been disclosed in the prior art for the purpose of producing fiber-forming resins (see, e.g., U.S. Pat. No. 3,701,702, which exemplifies compositions having very low polychloroprene content), laminating adhesives (see, e.g., U.S. Pat. No. 3,770,572 directed to latex adhesives comprising polychloroprene and ethylene copolymers), cured foamed compositions (see, e.g., U.S. Pat. No. 4,307,204), and elastomeric extrudable/moldable compositions (see U.S. Pat. No. 4,235,980 directed to terionomer blends).